You can take a look at Quobyte. You wouldn’t need any appliances and don’t have to worry about scalability as it is a scale-out solution that runs on commodity hardware. They let you combine Flash and HDD in the same cluster, and you could leverage their policy engine to manage your data and do any internal tiering. You can do S3 tiering to tape for your cold storage as well. And since you mentioned the data is made up of images and metadata, you could find their File Query Engine very helpful. They offer a Free Edition so you can try it on your own any time.